Some commonalities exist in this area, mainly of climate and culture.
Climate: Apart from the Himalaya, the climate is tropical, with monsoon in summer and dry winter. However, you have the extremes of this climate, i.e. in Western Pakistan monsoon is quite non-existent and in Southern India, it lasts for six months. Sri Lanka even has two monsoons, one in May, one in October/November.
Culture: The influence of historical Indian culture can be seen everywhere. Two of the main "world religions" have their origins within South Asia: Hinduism and Buddhism. A third, Islam, was introduced by Muslim invaders starting around the 7th century and rose to prominence during the Mughal Empire.
An additional layer of South Asian cultural unification derives from the influence of British culture, and especially the frequent and growing use of the English language, as a result of India having formed the "Jewel in the Crown" of the British Empire before Independence in 1947. The British impact on India is most well understood in the former capital, Kolkata
Population density: South Asia is one of the world's most densely populated regions - approximately 1.6 billion people (or roughly a quarter of humanity) make their home there. The average population density of 305 people per square km is 7 times the world average.
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